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Making Interest Interesting

I have been feeling in a bit of a creative rut lately, as though I am constantly borrowing other people's ideas without creating my own. Which, I mean, is fine. Why reinvent the wheel, right?

But maybe my wheel could have flashing lights, or fancy spokes.

Point is, I'm an unashamed keener, and having something that I created makes me feel as though I am working towards the potential I see in myself.

This week, and last, I am/was subbing for a previous university lecturer of mine, someone who really changed the way I see and approach math education.

To say I was nervous when I walked into the classroom would be understatement.

In Foundations 30 we are working on the finance unit. Compound interest, mortgages, stocks. Edge of your seat kind of stuff.

I had been provided with a quiz from the regular teacher, one where students were given a choice of rates, compound vs simple interest, and time frame. I thought it was very interesting as I hadn't seen a quiz like this before. Putting that in my back pocket for later use.

As I stared at my computer screen I began to wonder how I could make an activity for students that would test their knowledge on what makes larger compounded interest and what makes less compounded interest. I flipped between online activities and worksheets. Then I thought, what if I get them to make me a situation with the largest return possible? I looked back at the quiz, as it had sparked this idea. I began to write:

1, 10, 7, 9, 15, 3, 4, 8

Make me a scenario of compound interest, using the above numbers, and without repeating any, that will give me the greatest return on an investment.

Make me a scenario of compound interest, using the above numbers, and without repeating any, that will give me the lowest amount of interest.

Now I was excited. Would the kids think to combine the numbers? Would they try and multiply them? Would they even bite onto this idea, or just stop after making one number and say that it's good enough?

The next day, after showing them the equation for calculating compound interest (I had gotten them to calculate compound interest the long way the day before so that they could understand what compound interest is) I had them do a couple straight forward "would you rather" questions regarding compound and simple interest scenarios. This was to make sure they were comfortable using the equation.

Finally, I had them write down the numbers and then gave them the first prompt. It took them a minute to figure out what the question meant, but once they were good they started going. After a couple of walk around I saw that the largest number was about $300,000 return. I told them to go see if anyone else had gotten a number larger than theirs. Once other groups started to see what numbers were possible, that's when we started cooking with gas.

The groups started to try and get the largest number possible, having some healthy competition with one another, which was wonderful to see. Now I would call out the largest return I could see and students would quickly convene with their groups to try and get a larger number.

Eventually someone asked,

"Can I put the numbers together?"

I said, "I never said you couldn't. I just said you can't change the digits."

Now the numbers were getting even bigger, until we finally hit a million. At that point I told them that this was no longer an investment, but a loan, and I want the lowest return possible because I don't want to have to pay back a bunch of interest.

We went through the same sequence of events, with nice friendly competition fueling their engagement.

We went right up to the bell, and while I wish I would have gotten us to reconvene and discuss what we had noticed in regards to the equation and how the numbers affect the final interest, I was happy with how the lesson went. It felt as though the students not only learned something, as I heard many saying, "if we change n to a bigger number we will have it compound more" or "If we invest for more years we will get a bigger return" and "how big can our interest be? Can it be more than 100% do you think?", but also that they had fun and were engaged by their thinking.

I can't wait to use this again with students in the future and see how I can incorporate the thinking classroom in more lessons.

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